The present invention relates to a so-called sliding correlator employable in a receiver of a SSC (Spread Spectrum Communication) system for detecting a correlation between a spreading code included in the received signal and a corresponding code locally-generated in the receiver and, more particularly to correlators employing digital techniques.
Referring now to FIG. 1A, a transmitting unit 100 includes a PN (Pseudo random Noise) generator 3 which produces a PN code for application to a PN modulator 2. A signal containing data to be transmitted, is applied through an input 1 to PN modulator 2. Modulation of the data-containing signal with the PN code produces a spread spectrum communication signal (SSC signal) which has a bandwidth that is several times (at least 10) as wide as the bandwidth of the data-containing signal. The output of PN modulator 3 is applied to an input of a carrier modulator 5. A carrier generator 4 generates a radio-frequency (RF) signal for application to a second input of carrier modulator 5. The carrier RF signal is thus modulated by the SSC signal to produce a broadband signal centered on the frequency of the RF signal. The modulated RF signal is applied to a transmitting antenna 6, from which it is radiated.
In general, the PN signal produced by PN generator 3 appears to be random noise but, in fact, varies in a known repetitive pattern contained in the particular PN code being used. The pattern may be produced as a pattern of frequency changes, phase changes, timing changes, or other techniques which occur at a rate that is rapid compared to the modulation of the data signal entering at input 1. To a receiver that lacks knowledge of the PN code, the transmitted signal appears indistinguishable from noise. To a receiver having knowledge of the PN code, the repetitive nature of the PN code can be used to reconstruct the original signal.
Referring now to FIG. 1B, a receiving unit 200 includes a receiving antenna 7 for receiving the SSC signal transmitted from transmitting antenna 6 (FIG. 1A). The received signal is applied to an input of a correlator 8. A reference PN generator 9 produces a PN code for application to a second input of correlator 8. An output of correlator 8 is applied to an input of a demodulator 10. Demodulator 10, which contains a local oscillator and mixer (not shown) produces an output 11 which, upon the meeting of certain conditions, contains only the data which was applied to input 1 of FIG. 1a.
The conditions for correct demodulation of the data from the received signal require correlation between the PN code in the received signal and the PN code generated by reference PN generator 9. That is, the conditions are:
1. The PN code generated by reference PN generator 9 must be the same as the PN code generated by PN generator 2, and
2. The two PN codes must be aligned in time.
When these conditions are met, correlator 8 removes the PN code from the received signal to produce a result that is the same as would have been created if the data signal modulated the carrier frequency, without the additional modulation by the PN code. Demodulator 10 thus performs conventional demodulation of the signal it receives to extract the data contained in the signal.
There are two well-known conventional techniques for determining the correlation between the PN code in the received signal and the reference PN code generated in receiving unit 200. These techniques are the matched filter method and the sliding correlation method.
Japanese Patent Publication SHO64-11178, Japanese Patent Provisional Publication SHO58-131840 disclose the matched filter method. In the matched filter method, the received signal, containing the PN code plus the data, and the reference PN code are fed into a matched filter such as, for example, a convolver. In the convolver, the received and reference PN codes are compared with each other. The convolver generates a pulse signal based upon the coincidence of the PN codes, and on the degree of correlation between them.
The sliding correlation method employs a sliding correlator in receiving unit 200 for generating pulse signal having an amplitude depending on the degree of correlation between the PN codes. The sliding correlator adjusts the frequency, phase or timing of the reference PN code to maximize the amplitude of the pulse signal.
Referring to FIG. 2, a sliding correlator, employs a so-called delay-lock method in which the frequency of the reference PN code is adjusted to align the phase of the reference PN code with the phase of the PN code in the received signal.
The input signal from receiving antenna 7 is fed inputs of a pair of multipliers 12, 13. PN generator 3 contains a voltage controlled oscillator (VCO) 18 feeding a signal to a PN generator 19. The output of PN generator 19, PN1, is fed directly to a second input of multiplier 13. The output of PN generator 19 is delayed for a time "T" equal to half of a "chip" of the PN code in a delay circuit 20 to produce a delayed PN signal PN2 for application to a second input of multiplier 12. A chip is defined as a signal frequency output from a frequency hopping signal generator. Chip time signifies the time occupied by that single frequency. Chip time also signifies the period of a code clock.
The output of multiplier 12 is applied to an input of an envelope detector 14. Similarly, the output of multiplier 13 is applied to an input of an envelope detector 15. The output of envelope detector 14 is applied to a + input of a comparator 16. The output of envelope detector 15 is applied to a - input of comparator 16. The output of comparator 16 is applied to modulator 10 (FIG. 1B) and to an input of a loop filter 17. Loop filter 17 filters out the AC component in the signal at its input, and applies the remaining DC component in its input to a control input of VCO 18.
Referring now to FIG. 3, in general, one PN code consists of thirty one (31) chips. Output 1 of envelope detector 14 is defined as a "lagging" output, since it relies for detection on the delayed PN code PN2. Similarly, output 2 of envelope detector 15 is defined as a "leading" output, since it relies for detection on the undelayed PN code PN1. Output 1, as shown in FIG. 3, represents the output of envelope detector 14 as the phase of a chip of PN code passes through correlation with the delayed PN code PN2. Similarly, output 2 represents the output of envelope detector 15 as the PN code passes through correlation with undelayed PN code PN1. The output of comparator 16 is: EQU Vc=V2-V1
Where:
V2=Amplitude of output 2 and PA1 V1=Amplitude of output 1
When V2=V1=0, the PN code in the received is canceled at the output of comparator 16. The remaining signal includes only the original carrier signal plus the data signal.
The outputs of comparator 16 passes though a positive peak when output 1 passes through its peak, then through zero as outputs 1 and 2 pass though equality, and then through a negative peak as output 2 passes through its positive peak. It will be recognized that the DC component in the filtered output of comparator 16, when applied to VCO 18, will control the frequency and phase of VCO 18 to maintain the output of comparator 16 at a center frequency or tracking point approximately equal to zero. That is, the two components of the received PN code are subtracted from each other, leaving only the carrier and data signal for connection to demodulator 10. This is the point of correlation of the reference PN code and the PN code in the received signal.
Correlation by correlator 8 proceeds in two steps. First, when the correlation value, representing the degree of the coincidence between the received PN code and the reference PN code reaches a certain threshold value, a synchronizing state is established, in which the frequency of the reference PN code is approximately equal to the received PN code. In the next step, a tracking operation adjusts the phase of the reference PN code to a value, as described above, which is effective to cancel the PN code in the received signal, and to thus provide the carrier and data signals for final detection. The principle of the delay-lock method of correlation is described in detail in "Spread Spectrum System" written by R. C. Dixon and published by WILEY-INTERSCIENCE PUBLICATION.
The above-described sliding correlator is especially adapted for use with analog circuits including filters, amplifiers, and multipliers. However, techniques for fabricating semiconductor devices have improved to the point where elements for processing digital signals can be fabricated at low cost. The cost advantage, as well as opportunities for miniaturization and automated volume production, offered by digital techniques are such that it may be preferable to employ digital techniques in the correlator.
Digitized VCOs, such as, for example, an NCO (Numeral Controlled Oscillator), or a DDS (Direct Digital Synthesizer) may be used to control a correlator. However, such digital VCOs conventionally employ relatively long digital words, of perhaps 23 bits, for their control. Such long digital words undesirably increases the size of the correlator device.